1. Field of the Invention
The present invention relates to an angle detecting device, and to an exposure apparatus which employs the same, e.g., an exposure apparatus in which alignment between a mask and a wafer is performed using positioning light.
2. Description of the Related Art
In exposure apparatuses which are employed in the production of semiconductor chips, a mask on which a geometric pattern is formed and which serves as an original plate and a wafer substrate to be exposed must be aligned with a very high degree of accuracy.
FIG. 18 is a schematic view of a conventional exposure apparatus. In FIG. 18, a mask pattern for semiconductor chips is formed on a mask 1501. A resist is applied on a wafer substrate 1502 which may be a silicon wafer. A movable table 1503 is movable with the wafer substrate 1502 held thereon. An alignment optical system 1504 detects a shift in the position between the mask 1501 and the geometric patterns etched on the wafer substrate 1502. The mask 1501, the alignment optical system 1504, the movable table 1503 and other components are mounted on a base 1505. An illumination beam 1506 used in the exposure process may be synchrotron radiation.
FIG. 19 is an enlarged view of the essential parts of the exposure apparatus of FIG. 18. When an X-ray source is used as the illumination source, a mask pattern 1601 formed on the mask 1501 may be made of an X-ray absorber, such as gold. A resist 1603 is applied on an existing pattern 1602 previously etched into the wafer substrate 1502. Exposure must be conducted when the pattern 1601 on the mask 1501 is accurately aligned with the existing pattern 1602 on the wafer substrate 1502. To achieve this, the wafer substrate 1502 is registered with the mask 1501 such that a shift .delta..sub.1 between the mask pattern transfer position defined by the direction of the illumination beam 1506 and the existing pattern on the wafer is zero. Therefore, it is desirable that the referential optical axis for the measurements conducted by the alignment optical system 1504 coincide with the direction of the illumination beam 1506. Even if they do not coincide with each other, the shift determined by the direction of the illumination beam 1506 can be corrected in the following manner.
Assuming that .THETA. is the angle between a straight line 1605 which is the reference of the measurements conducted by the alignment optical system and a straight line 1604 indicating the direction of the illumination beam 1506 and that G is the distance (the proximity gap) between the wafer substrate 1502 and the mask 1501, a shift .delta..sub.2 measured by the alignment optical system is corrected based on a differential .delta. given by
.delta.=.THETA..multidot.G PA1 .delta..sub.1 +.delta..sub.2 =.delta. PA1 .epsilon.=.THETA..multidot..increment.g
so as to obtain the shift .delta..sub.1 determined by the direction of the illumination.
However, in this case, the proximity gap G must be a known factor. If an error .increment.g exists in the gap G, the following error .epsilon. cannot be corrected.
If .increment.g=2 .mu.m, .vertline..epsilon..vertline.&lt;0.002 .mu.m can be achieved only when .vertline..THETA..vertline..ltoreq.1 mrad.
FIG. 20 shows a conventional mechanism for detecting whether the optical axis of the radiation used for exposure coincides with the referential axis of the exposure apparatus. In this mechanism, two pin-hole plates 1701 and 1702 each having a pin-hole of a diameter d at the central portion thereof are disposed at an interval of a predetermined distance L in such a manner that the pin-holes are aligned with the referential axis of the exposure apparatus. The light which passes through the pin-hole plates 1701 and 1702 is detected by a detector 1703 which detects the shift of the optical axis of the exposure radiation from its output value.
FIG. 21 is a graph showing the relation between the intensity of light detected by the detector 1703 and the angle .THETA. between the optical axis of the exposure radiation and the referential axis of the exposure apparatus. The detector 1703 detects the maximum light intensity when the optical axis of the exposure radiation coincides with the referential axis, and the angle .THETA. is thereby reduced to zero. When an angle .THETA. exists the detected intensity of the light decreases accordingly. The light intensity curve of the detector 1703 is symmetrical to the aforementioned maximum value thereof.
The aforementioned conventional angle detecting mechanism for the exposure apparatus detects the maximum value when there is no angle, but detects two values when there is an angle. It is therefore impossible to determine both the shift of the optical axis and the direction of the shift thereof using the output value of the detector alone. This makes adjustment of the positioning of the exposure apparatus difficult. When the angle of incidence is to be obtained from the output of the detector, the angle detection mechanism must be moved over the entire range of angles so as to obtain the angle corresponding to the maximum output value. This is a troublesome task. Furthermore, when it is desired to enhance the detection accuracy, the value d/L (where L is the distance between the pin-hole plates, and d is the diameter of the pin-hole) must be reduced. However, reduction of the ratio d/L narrows the range of detectable angles or makes alignment of the exposure radiation with the referential axis of the exposure apparatus difficult.